It is known to measure the pressure of lubricating oil in an internal combustion engine, and to issue a warning when the measured oil pressure falls below a prescribed threshold value. See JP4235485B, for instance. The control system disclosed in this prior patent document issues a warning when the oil pressure falls below a prescribed threshold value provided that the engine is in operation, and the rotational speed of the engine is higher than a prescribed threshold value. The rotational threshold value is selected at a lowest possible level that can occur when the engine is in operation.
When measuring the oil pressure of an engine, it is necessary to take into account the rotational speed of the engine. When the rotational speed of the engine drops below a prescribed level, because the oil pressure is created by a pump actuated by the rotation of the crankshaft, the oil pressure drops. But this should be distinguished from the actual case of a drop in oil pressure that should be warned about. Therefore, it is essential to take into account the rotational speed of the engine when an oil pressure warning is to be made.
The rotational speed of an engine is typically measured by using a rotary encoder that counts crank pulses obtained by detecting the passing of projections formed on a crank pulser rotor attached to the crankshaft of the engine near a magnetic detector. Thus, the rotary encoder detects the rotational speed of the crankshaft according to the number of pulses counted during a prescribed sampling time. Since the measurement is based on the number of pulses preceding the current time point, as opposed to an instantaneous value, a certain time delay is inevitable in the measured value. In particular, when the rotational speed changes rapidly, a significant deviation from the actual rotational speed may occur owing to this time delay.